1. Field of the Application
The present application relates to the problem of cooling the lubrication circuit of a bypass turbomachine, more particularly to setting in place a cooler in the secondary flowpath of a bypass turbomachine for an optimized cooling of the lubrication circuit.
2. Description of Related Art
Various mechanical or electrical elements of a turbomachine must be lubricated and/or cooled, such as, for example, certain bearings, certain reducers, and the electrical equipment. To do so, a lubrication circuit is provided. The oil plays the role of a lubricant and also of a heat transfer fluid, which necessitates providing for the heat thus collected to be evacuated. Current turbojet engines generate more and more heat, particularly for the following reasons:                greater and greater weight of the bearing chambers;        integration of high-power reducers, particularly in jet prop engines, the engines referred to as “open rotor” equipped with a non-faired dual fan or, in conventional turbojet engines where the fan is made to rotate at a lesser speed than the low-pressure compressor for the purpose of improving the performance; and        integration of new equipment such as, for example, high-power starters/generators.        
For these reasons, the fuel-oil exchangers (FCOC for Fuel cooled Oil cooler) are saturated and require the addition of cooling capacity via air-oil exchangers (ACOC for Air cooled Oil cooler). Various solutions are available to ensure the evacuation of the heat from such exchangers as, for example, the use of one or several scoops on the fairing which causes an increase of the drag, or the tapping of the engine airflow, which causes an aerodynamic disturbance of the flow of the engine, and, consequently, a decrease of performance.
In order to minimize the aerodynamic disturbances, heat exchangers, referred to as surface ones, are developed. They replace a wall in contact with an air circulation in order to present a thermal exchange surface between the heat transfer fluid such as, for example, the oil of the lubrication circuit and the air (cooling source).
The patent document EP 2075194 A1 discloses a bypass turbojet engine with an air-oil exchanger arranged on the inner shroud of the secondary flowpath in the area of the separator nose, from the leading edge up to the vicinity of outlet guide vanes. A series of fins, parallel to one another and perpendicular to the nose surface, extend over the surface of the end of the separator nose. According to a first embodiment, the oil flow channels are in the thickness of the wall forming the nose, which imposes rather substantial construction constraints. According to another embodiment, the channels are constituted by the space delimited between two plates of a stack of three plates comprising two plates, parallel and slightly spaced apart, between which a third, undulated plate, is housed by being connected in a fluid-tight manner by each line forming the top of all the undulations. This sandwich-type structure, despite being light and strong, has the drawback of having a limited thermal exchange capacity. More generally, making this surface exchanger according to this teaching imposes particular constraints when making the wall of the separator nose. The manufacturing cost of the walls is high and setting them in place is restrictive. Furthermore, the thermal exchange capacity is limited due to the exchange surface, which is limited up to a certain distance downstream from the leading edge. The extension of the exchange surface at a greater distance from the leading edge could cause mechanical stability problems due to the excess weight engendered by the exchanger.
Surface exchangers, although they optimize the aerodynamic impact, are generally more complex to manufacture and to integrate than the exchangers referred to as compact. Indeed, they must conform geometrically to the surface which they replace and they represent additional loads (weights, thermal expansion . . . ) for the zone which they integrate.
The patent document EP 1916399 A2 discloses a bypass turbojet engine with an annular surface exchanger integrated in the outer wall of the secondary flowpath. The integration of the exchanger in the outer wall is a priori easier than in the inner wall. However, the major drawback of this arrangement is that the heat exchanger is subject to deterioration by outside objects which penetrate the turbojet engine. Indeed, the rotation movement of the fan breaks these outside objects into small pieces; the small pieces are then pushed by centrifugal force downstream of the fan toward the outside of the secondary flowpath.
Although great strides have been made in the area of axial compressors, many shortcomings remain.